Method for recognizing abnormal combustions in the cylinder of an internal-combustion engine

ABSTRACT

The speed for each cylinder is determined by means of two crankshaft positions z1, z2. The crankshaft positions z1, z2 are optimized in terms of the case of abnormal combustion. Abnormal combustion is recognized when, according to the selected definition of the speed difference ΔT, a limit value GW is not reached or is exceeded.

CROSS REFERENCE TO RELATED APPLICATION

A related application is International Application PCT/EP91/02140, filed12 Nov. 1991, and filed for entry into the U.S. national phasesimultaneously herewith.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to a process for recognizing abnormalcombustions in the cylinder of an internal-combustion engine.

2. Description of the Related Art

U.S. Pat. No. 4,697,561 discloses a method in which the speeds for eachcylinder are detected, during its work cycle, by means of the crankshaftpositions in which the crankshaft is at the lowest and the highest speedrespectively. The difference between these two speeds is a measure ofthe torque generated by the cylinder. This, when related to the averagedtorque of all the cylinders, gives an indication of the operatingefficiency of the cylinder in question, which is utilized for diagnosticpurposes.

In this method, the crankshaft positions having the lowest and thehighest crankshaft speed have to be determined experimentally on theengine test bench. In particular, they are dependent on the load and onthe speed and are stored in this dependence in the control unit forcarrying out the method.

Corresponding tests on the engine test bench have shown that it isdifficult to fix these crankshaft positions. Particularly for thepurposes of the diagnosis of abnormal combustions, it has not beenpossible, in specific load and speed ranges, to find any crankshaftpositions which would give a speed-difference signal which can beevaluated with sufficient accuracy.

SUMMARY OF THE INVENTION

The object of the present invention is, therefore, to develop the methodin such a way that the evaluation capability is improved.

The solution according to the invention is achieved by determining thespeed of the internal-combustion engine by means of two crankshaftpositions which are selected as different depending upon load and onspeed, wherein the two crankshaft positions are selected so that, inrelation to the case of abnormal combustion, as great a speed differenceas possible is obtained, and recognizing abnormal combustion when,according to the selected speed difference, a chronologically laterminus chronologically earlier rpm value or chronologically earlier minuschronologically later rpm value, either exceeds or does not reach alimit value. Advantageous developments of the invention are includeselecting the two crankshaft positions in relation to the case of acombustion misfire. The method preferably includes determining the speedfor each cylinder continuously over two crankshaft-position ranges andobtaining one of the two speeds respectively from these by averaging.Simplification is provided when the limit value is zero so that whetheror not the speed difference has exceed the limit value or has notreaching the limit value is recognized from the sign of the speeddifference. Preferably, the crankshaft positions are selected asdifferent in a cylinder-specific manner.

The invention proceeds from the knowledge that the problems ofevaluation arise as a result of the choice of the optimization strategyfor the crankshaft positions. In the state of the art, those crankshaftpositions in which the crankshaft is at the lowest and the highest speedrespectively are sought.

These crankshaft positions are determined during the normal operation ofthe internal-combustion engine, so that evidence of the torque generatedby it can be derived. In the event of faults occurring to a pronouncedextent in specific load and speed ranges, however, the speed differencedetermined in the two crankshaft positions is often no longersufficient.

According to the invention, therefore, this speed difference increasesby optimizing the crankshaft positions not in terms of normal operation,but in terms of abnormal combustion. The second crankshaft position inparticular is selected at a later stage than would correspond to thepoint of highest crankshaft speed during normal operation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail by means of the drawings. Inthese:

FIG. 1 shows a simplified block diagram of an arrangement for carryingout the method,

FIG. 2 shows a speed curve of the crankshaft of an internal-combustionengine to illustrate the method, and

FIG. 3 shows a flow diagram for carrying out the method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an internal-combustion engine B with a crankshafttransmitter KWG and with a camshaft transmitter NWG. The crankshafttransmitter senses a gearwheel which is connected to the crankshaft andwhich has a reference mark and a plurality of teeth. The crankshafttransmitter KWG each time supplies as an output signal one pulse pertooth and one reference pulse, distinguishable from this, at thereference mark. This output signal is fed to a control unit S which, bycounting the pulses, on the one hand determines the position of thecrankshaft within a revolution of 360° and on the other hand determinesthe speed by means of the time sequence of pulses per unit time.

The camshaft transmitter NWG senses a gearwheel which is connected tothe camshaft and which has a tooth over one half of its circumferenceand a gap over the other half of its circumference. The output signalfrom the camshaft transmitter NWG has two states, depending on whetherthe tooth or the gap is opposite the camshaft transmitter NWG. Thecontrol unit S determines the cylinder assignment and the crankshaftangle by means of this output signal together with the output signalfrom the crankshaft transmitter KWG. A camshaft transmitter NWG is usedbecause, for example, on a 6-cylinder engine, there is a periodicity oftwo respective revolutions and therefore the position of the crankshaftalone is not yet sufficient for a clear assignment in the ignitionsequence.

The control unit S has essentially the function of controlling theignition and injection for the internal-combustion engine B and ofperforming various diagnostic functions. For this purpose, varioussensors and actuators, which are not shown, are connected to the controlunit S. Such a control unit S therefore constitutes a conventionalengine control, such as is already put into practice at the present timein many vehicles.

One diagnostic function executed by the control unit S is therecognition of irregular combustions in one of the cylinders of theinternal-combustion engine B. As an illustration, the reciprocal speed Tof the crankshaft is plotted against the crankshaft position z in thegraph of FIG. 2. The reciprocal speed was selected because acorresponding signal is already present in the control unit S. Inrelation to a consideration of the speed, therefore, the conditions arereversed. Consequently, a rising edge thus signifies a slowing and afalling edge an acceleration of the crankshaft. A normal curve Nreproduces this trend for the normal operation of theinternal-combustion engine B. The sinusoidal curve results from thevarious work cycles of the internal-combustion engine B, the individualacceleration phases each being located after the ignition of a cylinderduring its work cycle. In FIG. 2, the top dead centers of the cylinders1, 5, 3, 6 are shown by way of example according to the ignitionsequence in a 6-cylinder engine.

The conditions which occur when proper ignition and combustion of themixture have not taken place during the work cycle of a cylinder areshown in FIG. 2 for the cylinder 1. A misfire curve A applies when nocombustion takes place at all. The crankshaft is therefore notaccelerated, but is slowed further as a result of the resistances of theengine. Only after the ignition of the next following cylinder 5 withproper combustion is the crankshaft then accelerated again.

The method according to the invention for recognizing such abnormalcombustions is described by means of the flow diagram of FIG. 3. Acorresponding program is filed in a diagnostic routine of the controlunit S and is executed continuously for each cylinder.

For this purpose, two crankshaft positions z1, z2 are fixed in relationto the respective top dead center . of a cylinder. These crankshaftpositions z1, z2 are conventionally located after the top dead center,differ in dependence on load and on speed and are optimized by means oftests. It is essential, here, that the normal case according to thenormal curve N is not used for the optimization, but the case ofabnormal combustion, in the example the case of a combustion misfireaccording to the misfire curve A.

In step S1 of FIG. 3, the air mass LM, the speed n and the cylindernumber of that cylinder which next reaches its top dead center aredetected. These quantities are also used for the purposes of calculatingthe ignition time and the injection time, so that they are alreadypresent in the control unit S and can be obtained from the correspondingprogram routines.

In step S2, the crankshaft positions z1, z2 are then taken fromcorresponding families of characteristics in dependence on the air massLM, the speed n and the cylinder number. The cylinder-specificdependence on the cylinder number is necessary in order to take intoaccount the vibration behavior of the crankshaft.

To reduce disturbing influences, the corresponding reciprocal speeds T1,T2 are not only detected once respectively in the crankshaft positionsz1 and z2, but in crankshaft-position ranges za, zb around thecrankshaft positions z1, z2. Finally, the reciprocal speeds T1 and T2corresponding to the reciprocal speeds in the two crankshaft positionsz1 and z2 are obtained from the averaging of the respective reciprocalspeed Ti detected thereby. This operation is carried out in steps S3, S4and S5, S6.

In step S7, the calculation of the difference ΔT from the reciprocalspeeds T1 and T2 then takes place. This difference ΔT is a directmeasure of the crankshaft acceleration brought about during propercombustion.

If the difference ΔT falls below a limit value GW, therefore, there isno combustion at all or faulty combustion. If this is so, a misfire isrecognized in steps S8 and S9.

In the method according to the invention, this difference ΔT is clearlygreater than in the state of the art, with the result that evaluationbecomes simpler and less susceptible to faults. As a comparison, FIG. 2shows two cases for the misfiring cylinder 1. According to theinvention, on the one hand the crankshaft positions z1 and z2 areoptimized in terms of the misfire curve A and on the other hand thecrankshaft positions z1' and z2' are shown optimized in terms of thenormal curve N are shown. As the resulting differences ΔT and ΔT'indicate, a virtual doubling occurs as a result of the optimizationaccording to the invention in terms of the misfire curve A.

The choice of the limit value GW determines the degree of abnormalcombustion which will still be recognizable. An especially simple methodfor recognizing complete combustion misfires would be, for example, apure sign evaluation of the difference ΔT. It can be seen from theexample of FIG. 2 that there is a negative difference ΔT in the case ofthe misfiring cylinder 1 and a positive difference ΔT in the case of thecombusting cylinder 5.

For a more sensitive evaluation of even only slightly abnormalcombustions, the limit value GW is set to a specific positive value.This also makes it possible to recognize as abnormal combustions which,it is true, provide acceleration contribution, but one which isinsufficient in comparison with that during normal combustion. Such afaulty combustion would give a curve trend which is between the misfirecurve A and the normal curve N.

Although other modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventors to embodywithin the patent warranted hereon all changes and modifications asreasonably and properly come within the scope of their contribution tothe art.

We claim:
 1. A method for recognizing abnormal combustions in a cylinderof an internal-combustion engine, comprising the steps of:determining aspeed of the internal-combustion engine at two crankshaft positionswhich are different depending upon load and on speed, said twocrankshaft positions defining a speed difference of the speed of theinternal combustion engine at the two positions, selecting said twocrankshaft positions so that, in relation to abnormal combustion, asgreat a speed difference as possible is obtained, and recognizingabnormal combustion when, according to a speed difference correspondingto said two crankshaft positions selected in said selecting step, achronologically later minus chronologically earlier rpm value orchronologically earlier minus chronologically later rpm value traversesa limit value.
 2. A method as claimed in claim 1, wherein said step ofselecting includes selecting said two crankshaft positions in relationto a combustion misfire.
 3. A method as claimed in claim 1, furthercomprising the steps of:continuously determining a speed for eachcylinder over two crankshaft-position ranges, and obtaining one of saidtwo speeds respectively from said continuously determined speeds byaveraging.
 4. A method as claimed in claim 1, wherein said limit valueis zero, and said step of recognizing is performed by determiningwhether said speed difference changes sign.
 5. A method as claimed inclaim 1, further comprising the step of:selecting crankshaft positionsas different in a cylinder-specific manner.